Electrophotographic materials



1964 K. DAMM ETAL 3,155,504

ELECTROPHOTOGRAPHIC MATERIALS Filed July 25, 1962 4 Sheets-Sheet 1 Min.

INVENTORS KLAUS DAMM, WOLF GES/E/P/CH GERHARD HEYL, FRANZ NISSL,

KARL SCHNURRBUSCH. BY

Nov. 3, 1964 K. DAMM ETAL 3,155,504

ELECTROPHOTOGRAPHIC MATERIALS Filed July 25, 1962 4 Sheets-Sheet 2 Min.

INVENTORS: KLAUS DAMM, WOLF GES/ER/CH, GERHARD HEYL, FRANZ N/SSL, KARL SCHNUFPRBUSCH. M

Nov. 3, 1964 K. DAMM ETAL 3,155,504

ELECTROPHOTOGRAPHIC MATERIALS Filed July 25, 1962 4 Sheets-Sheet 3 Sec.

INVENTORS KLAUS DAMM, WOLF GES/EP/CH, GEPHAPD HEYL, FRANZ N/SSL KARL SCHNURPBUSCH BY ILM 1X "T11; ATTO/PNEYY Nov. 3, 1964 K. DAMM ETAL 3,155,504

ELECTROPHOTOGRAPHIC MATERIALS Filed July 25, 1962 v 4 Sheets-Sheet 4 I5 45 Sec.

. INVENTOR$.' KLAUS DAMM, WOLF GES/ERICH, GEFPHARD HEYL, FRANZ N/SSL, KARL SCI-JNURRBUSCH. BY 1 M5 1 M UQA cw ATTOE'PNEM United States Patent 3,155,504 ELECTRUEHOTGGRAPHIC MATERIALS Klaus Datum, Cologne-Flittard, Wolf Gesierich, Leverkusen, Gerhard Heyl, tCologne-Stammheim, Franz Nissi, Munich, and Karl S'chnurrbusch, Cologne-Flattard, Germany, assiguurs to Agfa Aktiengeselisehaft,

Leverhusen, Germany, a corporation of Germany Filed .Fuly 25, 1962, Ser. No. 212,309 (Jlaims priority, application Germany Aug. 17, 1961 22 \Claims. (til. 96-1) This invention relates to improved electrophotographic materials, comprising a photoconductive layer of an insulating binding agent and zinc oxide dispersed therein, said zinc oxide has improved photoelectric properties.

Because of its photoelectric properties, zinc oxide is used inter alia for the production of photo-conductive layers for the production of electrophotographic images. Suitable eleetrophotographic materials comprise a photoconductive layer consisting of zinc oxide as the photoconductive compound dispersed in an insulating binding agent. The said photoconductive layer is arranged on a support being sufliciently conductive. To produce electrophotographic images, the photoconduotive layer is charged electrically and image-wise exposed, the charging being dissipated at the exposed areas, corresponding to the light exposure. The latent charged images thus formed can then be made visible by the aid of powder, liquid or aerosol developers, which are known per se.

Zinc oxide-types which are used for the production of such layers should have a highest possible light sensitivity and a lowest possible dark conductivity. The high light sensitivity is desirable, so that light sources of low light intensity and short exposure times can be used. The dark conductivity should be so low that no appreciable decrease in charging occurs during the period of time which is necessary for making the latent charging image visible.

With many applications it is necessary that the photoconductive layer maintains its charging for several hours or even days.

According to the invention, the most suitable Zinc oxide pigments for electrophotographic purposes are those which are prepared by the so-called French or indirect process in a high degree of purity.

It is among the objects of the present invention to provide a zinc oxide having improved physical respectively photoelectric properties as well as an electrophotographic material containing the said zinc oxide.

It has now been found that the photoelectric properties of light-conductive zinc oxides can be substantially improved if they are treated before dispersing in the solution of a suitable binding agent with non-filmforming organosilicon compounds of the following formulae:

wherein n is an integer from 1 to 3, p an integer from 2 to 6 and R represents hydrogen, an alkyl radical having up to carbon atoms, preferably methyl, an olefinically unsaturated alkyl radical, such as vinyl or an aryl radical, e.g., phenyl.

wherein m in an integer from 0 to 3, R stands for a hydrogen atom or an alkyl having up to 5 carbon atoms preferably methyl, or olefinically unsaturated alkyl such as vinyl, aryl such as phenyl, alkaryl or cycloalkyl radical and R" represents a lower alkyl radical having up to 5 carbon atoms, preferably methyl or ethyl.

Suitable compounds of Formula I are for example, octamethyl cyclotetrasiloxane, hexamethyl disiloxane, tetramethyl disiloxane, tetramethyl cyclotetrasiloxane, pentamethyl cyclotetrasiloxane or tetravinyl tetramethyl cyclotetrasiloxane. Formula II includes for example the following compounds: triisopropoxysilane, tributoxysilane, methyltrimethoxysilane, phenyldimethoxysilane, dimethyldiethoxysilane, methyldipropoxysilane, trimethylethoxysilane, vinyltriethoxysilane or the like.

It will be convenient here to state that the question is not settled whether or not the improvement of the zinc oxide is due to a chemical reaction between the zinc oxide and the organo silicon compounds. It is however not essential which reasons are responsible for the improvement of the zinc oxide. In some cases the organo silicon compounds may only be physically absorbed at the surface of the zinc oxide grain; in others a chemical surface reaction may occur on the zinc oxide gnain. Due to this uncertainty we want to define that where the terms reaction products or superficial reaction products are employed, we mean the zinc oxide grain con taining the 'organo silicon compound physically absorbed as well as the zinc oxide grain the surface of which has chemically reacted with the organo silicon compound.

Compounds having a boiling point of not higher than approximately 200 Celsius are preferred.

The best results are obtained if the zinc oxide is treated with organosilicon compounds in vapor form. For this purpose, the reaction vessel containing the Zinc oxide is brought to a temperature which is above the boiling point of the organosilicon compound and then the vaporized 'organosilicon compound is introduced while stirring.

The quantity of organosilicon compounds to be used is not critical and can lie between 1 and 40% by weight, calculated on zinc oxide. No adequate water-repellent eifect is produced below 1% and more than 40% does not provide any decisive advantages.

The light sensitivity and the dark resistance of photoconducting zinc oxides is substantially improved. The products of the process can therefore be used with advantage for the production of electrophotographic layers, for other semi-conductor layers and also for the production of toners for making latent charging images visible.

In the examples, the follwing methods have been employed for measuring the photoelectric properties.

For measuring the decrease in darkness, the electrophotographic material was charged by means of a corona discharge of 6 kv. and, by means of an instrument according to Schwenkhagen for measuring electrostatic field intensities, the change in the surface charging as a function of time was recorded. The measurement values obtained by this method are shown in diagrams (see FIGURES 1a and 1b). For testing the light sensitivity, the photoconductor layer was exposed to light at an interval of 15 seconds with light pulses of lux-sec. and also in this case the extent of the charging was recorded (see FIGURES 2a and 217).

Example 1 1000 g. of zinc oxide p.a. (product of the firm Merck) were contacted at C. with 100 g, of hexamethyl disiloxane. The zinc oxide was located in a heated roundbottom flask, which was equipped with a stirrer device. During the introduction of the organosilicon compound in vapour form into the reaction vessel, the zinc oxide was constantly circulated. After this treatment which was completed after about 3 hours, the reaction mixture was stirred for another 3 hours.

For the production of an electrophotographic layer, 450 g. of the siliconized zinc oxide were ground for two hours in a ball mill with 320 g. of a film-forming solid polysiloxane (silicone resin Bayer P 60% in toluene, and 500 g. of toluene, thereafter applied to a paper sheet lined with aluminum foil and dried. The photoelectric characteristics of a zinc oxide layer produced in this way is shown in the curves included in the graphs of FIGURES 1a and 2a of the accompanying drawing by comparison with untreated zinc oxide (Merck p.a.). Curves designated refer to untreated zinc oxide while those designated 1 relate to the treated zinc oxide layers.

In the following examples, the working time is the same as in Example 1.

Example 2 The reaction was carried out with 1000 g. of the zinc oxide described in Example 1 and 100 g. of octamethyl cyclotetrasiloxane at 190 C. The photoelectric properties of the product obtained by the process are likewise shown in FIGURES 1a and 2a. Curves designated 2 in these figures refer to this product.

Example 3 1000 g. of the zinc oxide described in Example 1 and 100 g. of triisopropoxysilane were used and the reaction temperature was 160 C. The effect of the organosilicon compound used in this example is shown in FIGURES 1a and 2a. Curves designated 3 in these figures refer to this product.

Example 4 2200 g. of ZnO (as described in Example 1) and 220 g. of methyltriethoxysilane were used and the reaction temperature was 160 C. FIGURES 1b and 2b again show the photoelectric effect by comparison with untreated zinc oxide. Curves designated 0 refer to products made with untreated zinc oxide while those designated 4 refer to the product of this example.

Example 5 1000 g. of ZnO (according to Example 1) were reacted at 150 C. with 100 g. of methyl hydrogen diisopropoxysilane. FIGURES lb and 2b show the photoelectric eifect. The curves designated 5 refer to the product of this example.

Example 6 A temperature of 130 C. was used for the treating 2200 g. of ZnO (according to Example 1) with 220 g. of dimethyl diethoxysilane. The resulting layer shows the photoelectric properties which are set out in FIGURES 1b and 2b. Curves designated 6 refer to the product of this example.

By adding sensitizers to the photoconducting layer, the spectral sensitivity thereof can be increased. All dyestuffs usual for this purpose in electrophotographic practice are suitable as sensitizers in conjunction with the process according to the invention.

Example 7 An electrophotographic material is produced according to Example 1 and electrostatically charged by means of the corona discharge device and imagewisely exposed.

For the development of the latent charging image, a filtered solution of 500 mg. of Crystal Violet (Colour Index 42555 Basic Violet 3) in 10 ml. of water is used. The electrophotographic image support is for this purpose tensioned on a roller with a thickness of about 10 cm. and rolled over a sand surface which is formed into a paste with the dyestutf solution and which is spread out fiat and doctored. The grain size of the sand is between and 50a. A sharp, practically grainless, violet colour image is obtained.

Example 8 An electrophotographic material according to the British patent specification 896,610 is produced as follows:

200 g. of a polyester prepared by vacuum esterification of 3 mols of phthalic anhydride, 3 mols of trimethylol propane and 1 mol of pentaerythritol (containing 11.8% by weight of hydroxyl groups, acid number 2) have added thereto at 140-170 C. 5 mols of cyclohexyl isocyanate. The reaction mixture is then heated for 2 hours at 170 C. The resin obtained has a softening point of 94 C. and is easily soluble in acetone, ethanol, ethylacetate and methylglycol acetate. This resin is dissolved in 1000 cc. of alcohol. 600 g. of zinc oxide prepared according to Example 1 are incorporated into this resin solution by stirring and the mixture is treated for several hours in a ball mill. It is then cast onto paper and dried at room temperature.

An electrophotographic image is produced as follows:

At first the electrophotographic material is charged by means of the corona discharge and thereafter exposed through the original to be reproduced. The latent image obtained thereby is developed by applying the pigment an usual developer powder carrying an electrostatic charge which is opposite to that of the photoconductive layer. The resulting powder image is fixed by melting the powder which is image-wisely distributed.

Although we have referred in the examples to the use of silicon resins as insulating binding agents for the electrophotographic layer, we now state that also other suitable binding agent can be used as for example alkyd resins or those described in the British specifications 834,- 502, 896,610 or published German applications A 25,565 IVa/57b (DAS 1,040,900) and R 17,949 Iva/57b.

The electrophotographic layers according to the invention may be sensitized by any suitable sensitizing dye known per se, e.g., dyestuffs of the triphenylmethane, cyanine or phenol sulphon phthalein series such as Fluorescein (C.I. 45350 Acid Yellow 73), Bengal Rose GTO (CI. 45435 Acid Red 93), Thioflavine T (CI. 49005 Basic Yellow 1), Acridine Orange NO (CI. 46005 Basic Orange 14), Brilliant Green (CI. 42040 Basic Green 1) or the sensitizers described in British Patent No. 874,133. The CI. numbers and the dye names in the parentheses refer to the number and name appearing in the Colour Index of The Society of Dyers and Colourists, Second Edition, London, 1958.

What we claim is:

1. A process for improving the photoelectric properties of zinc oxide which comprises adding zinc oxide the vapors of an organosilicon compound selected from those having the following formulae:

wherein n is an integer from 1 to 3, J an integer from 2 to 6 and m an integer from 0 to 3, R represents a member selected from the class consisting of hydrogen, alkyl having up to 5 carbon atoms, olefinically unsaturated alkyl and aryl, R stands for a radical selected from the group consisting of hydrogen, alkyl having up to 5 carbon atoms, olefinically unsaturated alkyl, aryl, alkaryl and cycloalkyl and R" represents a lower alkyl radical having up to 5 carbon atoms.

2. A process according to claim 1 in which the organosilicon compound is hexamethyl disiloxane.

3. A process according to claim 1, in which the organosilicon compound is octamethyl cyclotetrasiloxane.

4. A process according to claim 1, in which the organosilicon compound is triisopropoxysilane.

5. A process according to claim 1, in which the organosilicon compound is methyltriethoxysilane.

6. A process according to claim 1, in which the organo silicon compound is methyl hydrogen diisopropoxysilane.

7. A process according to claim 1, in which the organosilicon compound is dimethyl diethoxysilane.

8. In a process for the production of electrophotographic reproductions by electrostatically charging an electrophotographic layer being of zinc oxide in an electrically insulating film-forming binding agent, exposing the charged layer to an optical image to electrically discharge the layer in accordance with its exposure to light, and then developing the resulting electrostatic image, the improvement according to which the electrophotographic layer is made with zinc oxide which has been treated 5 with an organosilicon compound selected from those having the following formulae:

wherein n is an integer from 1 to 3, p an integer from 2 to 6 and m an integer from to 3, R represents a member selected from the class consisting of hydrogen, alkyl having up to carbon atoms, olefinically unsaturated alkyl and aryl, R stands for a radical selected from the group consisting of hydrogen, alkyl having up to 5 carbon atoms, olefinically unsaturated alkyl, aryl and cycloalkyl and R" represents a lower alkyl radical having up to 5 carbon atoms.

9. The combination of claim 8 in which the organosilicon compound is triisopropoxysilane.

10. The combination of claim 8 in which the organosilicon compound is methyltriethoxysilane.

11. The combination of claim 8 in which the organosilicon compound is methyl hydrogen diisopropoxysilane.

12. The combination of claim 8 in which the organosilicon compound is dimethyl diethoxysilane.

13. An electrophotographic layer of zinc oxide dispersed in an electrically insulating binding agent wherein said zinc oxide has been treated with the vapors of an organosilicon compound selected from those having the following formulae:

wherein n is an integer from 1 to 3, p an integer from 2 to 6 and man integer from 0 to 3, R represents a member selected from the class consisting of hydrogen, alkyl having up to 5 carbon atoms, olefinically unsaturated alkyl and aryl, R' stands for a radical selected from the group consisting of hydrogen, alkyl having up to 5 carbon atoms, olefinically unsaturated alkyl, aryl, alkaryl and cycloalkyl and R" represents a lower alkyl radical having up to 5 carbon atoms.

14. An electrophotographic layer according to claim 13, wherein the organosilicon compound is triisopropoxysilane.

15. An electrophotographic layer according to claim 13, wherein the organosilicon compound is methyltriethoxysilane.

16. An electrophotographic layer according :to claim 13, wherein the organosilicon compound is methyl hydrogen diisopropoxysilane.

17. An electrophotographic layer according to claim 13, wherein the organosilicon compound is dimethyl diethoxysilane.

18. The combination of claim 8 in which the treatment of the zinc oxide was efiected with the vapors of the organosilicon compound.

19. The combination of claim 8 in which the organosilicon compound is hexamethyl disiloxane.

20. The combination of claim 8 in which the organosilicon compound is octamethyl cyclotetrasiloxane.

21. An electrophotographic layer according to claim 13, wherein the organosilicon compound is hexamethyl disiloxane.

22. An electrophotographic layer according to claim 13, wherein the organosilicon compound is octamethyl cyclotetrasiloxane.

References Cited in the file of this patent UNITED STATES PATENTS 2,494,920 Warrick Ian. 17, 1950 2,579,332 Nelson Dec. 18, 1951 2,610,167 Te Grotenhuis Sept. 9, 1952 2,615,006 Lane Oct. 21, 1952 3,052,539 Grieg Sept. 4, 1962 3,052,540 Grieg Sept. 4, 1962 

8. IN A PROCESS FOR THE PORDUCTION OF ELECTROPHOTOGRAPHIC REPRODUCTIONS BY ELECTROSTATICALLY CHARGING AN ELECTROPHOTOGRAPHIC LAYER BEING OF ZINC OXIDE IN AN ELECTRICALLY INSULATING FILM-FORMING BINDING AGENT, EXPOSING THE CHARGED LAYER TO AN OPTICAL IMAGE TO ELECTRICALLY DISCHARGE THE LAYER IN ACCORDANCE WITH ITS EXPOSURE TO LIGHT, AND THEN DEVELOPING THE RESULTING ELECTROSTATIC IMAGE, THE IMPROVEMENT ACCORDING TO WHICH THE ELECTROPHOTOGRAPHIC LAYER IS MADE WITH ZINC OXIDE WHICH HAS BEEN TREATED WITH AN ORGANOSILICON COMPOUND SELECTED FROM THOSE HAVING THE FOLLOWING FORMULAE: 